django orm

正查询和反查

呵呵

https://www.cnblogs.com/liwenzhou/p/8660826.html

从有外键关联的表中查询没有外键的表 ---正查询; 反之反查询.比如如下字段:

class Author(models.Model):
    name = models.CharField(max_length=32)
    age = models.IntegerField()
    email = models.CharField(max_length=32)
    author_detail = models.OneToOneField(null=True,to="AuthorDetail", on_delete=models.CASCADE,related_name="author")


class AuthorDetail(models.Model):
    address = models.CharField(max_length=32)
    telephone = models.IntegerField()

正查询

使用author对象查询AuthorDetail对象信息,则是正查. 反之反查询

正查询: 如下: ad指AuthorDetail对象

对象查询:

author = Author.objects.get(id=1)
author.name
'金庸'
author.ad.address
'香港'

类名查询

正查询跨表获取某个字段,注意跨表时使用字段名称,而不是表名称:

Author.objects.filter(id=1).values("author_detail__address")
(0.000) SELECT `blog_authordetail`.`address` FROM `blog_author` LEFT OUTER JOIN `blog_authordetail` ON (`blog_author`.`author_detail_id` = `blog_authordetail`.`id`) WHERE `blog_author`.`id` = 1 LIMIT 21; args=(1,)
(0.000) SELECT `blog_authordetail`.`address` FROM `blog_author` LEFT OUTER JOIN `blog_authordetail` ON (`blog_author`.`author_detail_id` = `blog_authordetail`.`id`) WHERE `blog_author`.`id` = 1 LIMIT 21; args=(1,)
<QuerySet [{'author_detail__address': '香港'}]>
(0.000) SELECT @@SQL_AUTO_IS_NULL; args=None
(0.000) SELECT @@SQL_AUTO_IS_NULL; args=None
(0.000) SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED; args=None
(0.000) SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED; args=None
(0.000) SELECT `blog_authordetail`.`address` FROM `blog_author` LEFT OUTER JOIN `blog_authordetail` ON (`blog_author`.`author_detail_id` = `blog_authordetail`.`id`) WHERE `blog_author`.`id` = 1 LIMIT 21; args=(1,)
(0.000) SELECT `blog_authordetail`.`address` FROM `blog_author` LEFT OUTER JOIN `blog_authordetail` ON (`blog_author`.`author_detail_id` = `blog_authordetail`.`id`) WHERE `blog_author`.`id` = 1 LIMIT 21; args=(1,)

Author.objects.get(id=1).author_detail.address
(0.016) SELECT `blog_author`.`id`, `blog_author`.`name`, `blog_author`.`age`, `blog_author`.`email`, `blog_author`.`author_detail_id` FROM `blog_author` WHERE `blog_author`.`id` = 1 LIMIT 21; args=(1,)
(0.016) SELECT `blog_author`.`id`, `blog_author`.`name`, `blog_author`.`age`, `blog_author`.`email`, `blog_author`.`author_detail_id` FROM `blog_author` WHERE `blog_author`.`id` = 1 LIMIT 21; args=(1,)
(0.000) SELECT `blog_authordetail`.`id`, `blog_authordetail`.`address`, `blog_authordetail`.`telephone` FROM `blog_authordetail` WHERE `blog_authordetail`.`id` = 1 LIMIT 21; args=(1,)
(0.000) SELECT `blog_authordetail`.`id`, `blog_authordetail`.`address`, `blog_authordetail`.`telephone` FROM `blog_authordetail` WHERE `blog_authordetail`.`id` = 1 LIMIT 21; args=(1,)
'香港'

反查询

方式一: 对象.类名_set.all()

class Article:

　　tags = models.ManyToManyField(Tags, blank=True)

class Tags(models.Model):

""" 标签 Tag 也比较简单，和 Category 一样。 再次强调一定要继承 models.Model 类！ """

　　name = models.CharField('标签', max_length=100)

反查询即从Tags查询Article对象信息:

tags = Tags.objects.get(id=1)
out:<Tags: java标签1>
tags.article_set
out:<django.db.models.fields.related_descriptors.create_forward_many_to_many_manager.<locals>.ManyRelatedManager object at 0x00000157FF7B16A0>
tags.article_set.get(id=1)
out: <Article: java基础>

方式二:使用使用relate_name

作用: 用于反向查询时使用，代替小写表名__set,比如：related_name='publisher'；

# 验证查询操作
class Book(models.Model):
    nid = models.AutoField(primary_key=True)
    title = models.CharField(max_length=32, default=None)
    create_time = models.DateTimeField(auto_now=True)
    price = models.DecimalField(decimal_places=2, max_digits=8, default=None)
    publisher = models.ForeignKey(to="Publish", on_delete=models.CASCADE,related_name='publisher')
    authors = models.ManyToManyField(to='Author',null=True,related_name='authors')
    reader=models.CharField(max_length=12,null=True)

    def __str__(self):
        return str(self.nid) + ':' + self.title


class Publish(models.Model):
    nid = models.AutoField(primary_key=True)
    name = models.CharField(max_length=32)
    email = models.CharField(max_length=32)

则反查询值:

 跨表查询对象----对象查询

p = Publish.objects.get(nid=1)
p.name
'北京文化'
p.publisher.all()
<QuerySet [<Book: 1:book_0>, <Book: 2:book_1>, <Book: 3:book_2>, <Book: 4:book_3>, <Book: 5:book_4>, <Book: 6:book_5>, <Book: 7:book_6>, <Book: 8:book_7>, <Book: 9:book_8>, <Book: 10:book_9>, <Book: 11:book_10>, <Book: 12:book_11>, <Book: 13:book_12>, <Book: 14:book_13>, <Book: 15:book_14>, <Book: 16:book_15>, <Book: 17:book_16>, <Book: 18:book_17>, <Book: 19:book_18>, <Book: 20:book_19>, '...(remaining elements truncated)...']>

 跨表查询对象中的字段-----类名查询

Publish.objects.filter(nid=1).values("publisher__title")
(0.000) SELECT `blog_book`.`title` FROM `blog_publish` LEFT OUTER JOIN `blog_book` ON (`blog_publish`.`nid` = `blog_book`.`publisher_id`) WHERE `blog_publish`.`nid` = 1 LIMIT 21; args=(1,)
(0.000) SELECT `blog_book`.`title` FROM `blog_publish` LEFT OUTER JOIN `blog_book` ON (`blog_publish`.`nid` = `blog_book`.`publisher_id`) WHERE `blog_publish`.`nid` = 1 LIMIT 21; args=(1,)
<QuerySet [{'publisher__title': 'book_0'}, {'publisher__title': 'book_1'}, {'publisher__title': 'book_2'}, {'publisher__title': 'book_3'}, {'publisher__title': 'book_4'}, {'publisher__title': 'book_5'}, {'publisher__title': 'book_6'}, {'publisher__title': 'book_7'}, {'publisher__title': 'book_8'}, {'publisher__title': 'book_9'}, {'publisher__title': 'book_10'}, {'publisher__title': 'book_11'}, {'publisher__title': 'book_12'}, {'publisher__title': 'book_13'}, {'publisher__title': 'book_14'}, {'publisher__title': 'book_15'}, {'publisher__title': 'book_16'}, {'publisher__title': 'book_17'}, {'publisher__title': 'book_18'}, {'publisher__title': 'book_19'}, '...(remaining elements truncated)...']>
(0.000) SELECT @@SQL_AUTO_IS_NULL; args=None
(0.000) SELECT @@SQL_AUTO_IS_NULL; args=None
(0.000) SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED; args=None
(0.000) SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED; args=None
(0.000) SELECT `blog_book`.`title` FROM `blog_publish` LEFT OUTER JOIN `blog_book` ON (`blog_publish`.`nid` = `blog_book`.`publisher_id`) WHERE `blog_publish`.`nid` = 1 LIMIT 21; args=(1,)
(0.000) SELECT `blog_book`.`title` FROM `blog_publish` LEFT OUTER JOIN `blog_book` ON (`blog_publish`.`nid` = `blog_book`.`publisher_id`) WHERE `blog_publish`.`nid` = 1 LIMIT 21; args=(1,)

related_name和related_query_name区别：

related_name：用于代替： 表名_set

related_query_name: 反向查询时用来代替表名字

 备注:

 具体的对象没有update()方法,只有queryset类型才有

 总结:

 

 多对多三种方式

1) 使用django创建第三张表:

   　优点: 查询方便,可以利用djaogo的正向和反向查询

 　  缺点: 第三张表中无法添加多余字段　　　

2)自己创建第三张表,即自己维护多对多关系,但是这样查询也需要自己做关联

　　优点:任意定义第三张表

 　　缺地:查询需要自己做关联

3)自己创建第三张表,但是查询时保留django的正向和反向查询

总结:

方式3)如下:

 自己创建第三张关系表

 

查询技巧 

查询某个字段

 分组

技巧: 

  models.Employee.object.all()  ---等价于: select * from Employee

  models.Employee.object.values("name","age")  ---等价于 select name,age from Employee

annotate函数:

   A: annotate函数以前一个values()中的字段进行分组,以后一个values()函数进行挑选字段

   B: 碰到反查 :表名__字段名    ---对应sql : inner join 

   C: 碰到正查 :关联字段名__字段名    ---对应sql : inner join 

　D:如果是Employee.object.all() .annotat()则是按照表的id进行group by

实例2:

 

　　如下: